1. Field of the Invention
The present invention relates to a thin solar cell and to a method for producing the same and, more particularly, to a thin solar cell of the type having a body composed essentially of one or more photoactive semiconductor layers, such as gallium arsenide, with the semiconductor body having on its frontal light entrance face a grid-shaped contact, an anti-reflection layer and a cover glass, and on its rear face a rear contact.
2. Description of the Related Art
High efficiency thin solar cells are presently a target of development all over the world since the ratio of performance to weight is important for use in satellites. Some solar cells are composed of a direct bandgap semiconductor material, such as gallium arsenide, which responds to the absorption of light within a depth of a few .mu.m. Direct bandgap semiconductors of a few .mu.m in thickness are thus able to electrically provide the full function of a solar cell. Other solar cells are composed of an indirect bandgap semiconductor material, such as silicon, which requires a layer of about 70 .mu.m for complete absorption of light. Solar cells made of GaAs exhibit greater efficiencies and better radiation resistance. The use of GaAs solar cells is therefore more advantageous for use with low-orbit satellites. One drawback, however, in using GaAs over Si in solar cells is that for the same thickness of the solar cell, the GaAs cell has a substantially greater weight than the Si cell.
In a GaAs solar cell and in solar cells composed of semiconductors of the III-V or II-VI groups of the Periodic Table of Elements, all of the photoelectrically sensitive layers are produced in an epitaxy process (growing of several differently doped and composed crystalline layers on a carrier substrate). The substrate on which the photoactive layers are produced, inter alia, has no photoelectric function and usually serves only for the production of these layers and to impart stability to the semiconductor body. It is possible to remove layers, that usually are unnecessary for the photoelectric function and undesirable because of their added weight (particularly if used in satellites), partially or in their entirety before the solar cells are put into operation. In this connection, GaAs and other compound semiconductors in which the photoelectric layers are produced in an epitaxial process, offer particular possibilities for separating the substrate layer from the epitaxy in order to produce thin semiconductor layers. A few of the separation methods include:
(1) the CLEFT process for epitaxial layers; the CLEFT process is described in Appl. Phys. Lett. 37(6), Sep. 15, 1980, pages 560-562;
(2) the lift-off technique by use of a chemically selectively-etchable epitaxially grown intermediate layer;
(3) an etching technique in which the substrate is etched away down to an epitaxially grown stop layer;
(4) polishing and/or lapping and/or chemically etching away the substrate down to the photoactive semiconductor layers can also be effected for layers that were not produced by epitaxial processes.
However, the complete or partial removal of the substrate is not a required part of the present invention. A thin solar cell can also be a cell in which the semiconductor body, comprising one or more photoactive semi-conductor layers, due to its low thickness and its concomitant fragility, can no longer be sufficiently manipulated without the presence of a stabilizing carrier. In existing solar cell devices, the attachment of connectors to interconnection pads on the thin semiconductor body by bonding, soldering, or other thermally or mechanically stressful processes in order to interconnect adjoining solar cells is very difficult because of the real danger of breakage involved.
The P/N junctions of most semiconductors and, for direct bandgap semiconductors, usually also the so-called window layer (e.g. AlGaAs for GaAs solar cells) required to reduce the high surface recombination are sensitive to environmental influences. The edges of GaAs solar cells and most other solar cells made of a direct bandgap semiconductor material must therefore be specially protected (passivated), at least as long as they are subjected to these influences. For GaAs solar cells, for example, this is usually done by etching a "mesa" trough around the active surface from the front through the photoactive layer with subsequent passivation by means of a dielectric medium (e.g. an anti-reflection layer) and sawing off the edge. This step includes a solar cell active area definition where the size of the photoelectrically active surface of the solar cell is determined.